1. Field of the Invention
The field of the invention relates to a composition for depositing precious metals on conductive substrates and processes utilizing such compositions.
2. Description of the Prior Art
Depositing of precious metals on to substrates has long been used commercially because the deposits provide desired characteristics, including, attractive appearance, high electrical conductivity, corrosion resistance and good soldering properties.
One of the most common precious metal plating electrolytes used is cyanide based; however, because of cyanide's toxicity, it causes problems in the electroplating working environment and associated waste treatment systems. Many cyanide-free precious metal electroplating systems have been devised to avoid these problems but sometimes the deposits produced from these non-cyanide baths are coarse and do not have as bright an appearance as deposits from cyanide based systems.
Another problem associated with precious metal plating solutions is the tendency for such solutions to immersion plate on active base metal substrates.
Immersion plating (also called displacement plating or substitution plating) occurs when an aqueous solution of a more noble metal ion is contacted with a less noble (more active) metal surface. The more noble ion tends to be reduced to elemental metal by electron donation from the less noble (more active) metal which as a result becomes itself oxidized to an ionic state (e.g., aqua-cation, soluble or insoluble metal oxide).
Metal deposits produced by immersion plating processes are typically limited to relatively low deposit thickness, as contact between the more active metal surface and the more noble metal ion is progressively decreased by the growing immersion layer. When the precious metal layer grows to a non-porous thickness, then the immersion plating stops.
When immersion plating is allowed to proceed in an uncontrolled manner, then a non-adherent metal deposit is obtained. It is advantageous to have cyanide-free precious metal plating solutions which do not operate with uncontrolled immersion plating, as controlled immersion deposits allow for precious metal coatings with superior physical characteristics.
It is known that the addition of certain organic compounds to precious metal plating solutions can usefully control the immersion process. By variation of the addition agents used, one can either control immersion plating to produce bright and adherent precious metal deposits or one can, with certain types of addition agents, completely prevent the immersion deposit from forming.
A number of publications have disclosed the use of organosulfur compounds and/or carboxylic acids in low-cyanide or cyanide-free silver electroplating solutions, and some of these publications address the problem of uncontrolled immersion plating.
For instance, U.S. Pat. No. 4,614,568 discloses a low-cyanide silver electroplating solution which contains a cyclic thioureylene compound additive known to prevent the deposition of silver by displacement reaction.
Also, U.S. Pat. No. 4,247,372 discloses a low-cyanide silver electroplating solution which contains a mercaptan compound additive able to prevent the deposition of silver by displacement reaction.
In addition, U.S. Pat. No. 4,452,673 discloses a low-cyanide silver pretreatment bath and Japanese Patent Application 57-131382 discloses a low-cyanide silver electroplating solution which contains a dithiocarbamic acid or thiosemicarbazide additive able to prevent the deposition of silver by displacement reaction.
Japanese Patent Application 03 061393 published Mar. 18, 1991, discloses a cyanide-free silver electroplating solution which contains a thiocarbonyl compound.
Natarajan (Metal Finishing, February '71, pg.51-56) has surveyed a number of cyanide-free formulations some of which contain completing organosulfur compounds and/or complexing carboxylic acids.
U.S. Pat. No. 4,478,692 describes aqueous electroplating solutions containing soluble palladium compounds and silver compounds, the solutions being capable of depositing a Ag/Pd alloy. Both the palladium and silver compounds may be salts of an alkanesulfonic acid. These silver and/or palladium salts are combined with an acid, which may be an organosulfonic acid, in an amount sufficient to keep the metal compounds in solution during the plating operation.
Kondo et al., Metal Finishing, Oct. 1991, pp. 32-36 describe an aqueous plating solution of silver methanesulfonate, potassium iodide and N-(3-hydroxy-1-butylidene)-p-aminobenzenesulfonic acid (HBPSA). A substantial amount of potassium iodide is a necessary component of this cyanide-free formulation in order to produce a silver electrodeposit on copper with a fine grain structure and appearance.
Japanese patent publication 96/41,676 discloses noble metal electroplating baths free from cyanides containing noble metal ions of alkanesulfonic acids and nonionic surfactants. The applicant states that the coatings formed show almost the same crystalline compactness as do coatings plated from cyanide-containing baths.
The present invention seeks to obtain the advantages of avoiding the above stated problems and other difficulties encountered in the related art.
This invention is distinct from the prior art in that it permits cyanide-free and halogen-free precious metal plating by taking advantage of the high solubility, unique properties, ease of formulation and ease of waste treatment associated with the precious metal salts of the alkanesulfonic acids, alkanesulfonimides and/or alkanesulfonamides; and this invention discloses solution compositions that can, if desired, completely prevent immersion plating.
These and other advantages are obtained according to the present invention which is the provision of a process and composition of matter that substantially obviates one or more of the limitations and disadvantages of the described prior processes and compositions of matter of the related art.